The invention concerns equipment for monitoring an elevator drive control and, in particular, equipment for monitoring a safety circuit of an elevator drive control.
In the case of present day elevator installations with frequency converter drives and microprocessor controls, mainly electromechanical contactors are used for the monitoring of the safety circuit and the consequential actions connected therewith, such as the brake actuation, the switching-on and switching-off of motor current and the loading of the intermediate circuit of the frequency converter with a defined switching-on current.
With electromechanical relays or also contactors, the mechanical contacts wear in use. Furthermore, contactors or relays cause appreciable noise emissions, which prove to be disturbing particularly in the case of elevator installations in residential or commercial buildings, during switching operations. Finally, contactors and relays require appreciable financial expenditure also by reason of their limited service life and frequent exchange.
Disadvantages also result due to the manner of operation of the safety circuit. Until today, the checking or the detection of the state of the safety circuit was performed by means of electromechanical contactors or relays. These contactors or relays in that case serve as sensors. However, this entails the following diverse disadvantages in an alternating current safety circuit:
Very long, parallelly laid electrical lines occur in an elevator installation. Due to the capacitance between the conductors, alternating voltage can be transmitted from one conductor to the other. Due to this effect, the mains voltage can be coupled into the safety circuit. This can have the consequence that contactors or relays do not drop off when a safety contact opens in the safety circuit, because the drop-off voltage in the case of alternating current contactors or relays is about one tenth of the attraction voltage. PA0 The same can happen when the voltage of the safety circuit is transmitted from one conductor of the safety circuit to a safety contact on the return conductor. PA0 Alternating current contactors or relays need a large switching-on current. In the case of a long safety circuit, the internal resistance is so great that special measures are required for voltage adaptation for the reliable switching-on. PA0 The operating voltage of the safety circuit is mostly in the range of 110 to 230 volts. For that reason, a protection against contact is required at all accessible places. PA0 The service life of the contactors and relays is greatly restricted by reason of the mechanical wear. PA0 The direct current leads to wear at the contact transitions of the safety contacts due to material migration.
Equally, disadvantages result in the case of a direct current safety circuit:
A monitoring device for a control device for elevator installations and conveying installations, which is provided with an electronic and testable switching device, which comprises a sensor and is initiatable without contacts and with the aid of which the state of the sensor is detectable, is shown in the European patent document EP-0 535 205. These contactless switching devices are to be used, for example, for the monitoring of the door latches.
In the case of the monitoring equipment described above, switching devices are used, which indeed eliminate the disadvantages of electromechanical switches, but are more expensive by a multiple, so that use is not worthwhile on cost grounds. Furthermore, this monitoring equipment requires complex electrical circuitry. Due to the capacitive cross talk, no loop can be formed in the case of longer electrical lines as is the case for a safety circuit for elevator installations. At the end of a line that can extend over several contacts, a signal converter must be used in order that the signal running back parallelly to the source signal can be distinguished from the source signal possibly coupled in capacitively.